Project Summary: Obesity and its associated metabolic diseases are major health problems around the world. Although the reasons for the rapid increase in rates of obesity and overweight, particularly in children, are multifactorial, it is clear that neural circuits in the brain play a major role in sensing energy stores and regulating energy balance. These neural pathways are candidate targets for the development of new pharmacotherapies aimed at reversing the obesity epidemic and therefore it is essential that we understand their function in detail, including the complete map of interconnections with other neural systems and their utilization of multiple neurotransmitters and intracellular signalling pathways. This project focuses on a key component of the brain[unreadable]s energy balance circuitry, the proopiomelanocortin (POMC) neurons located in the hypothalamus and brainstem. Genetic deletion of POMC function from the brain results in an obesity and metabolic syndrome characterized by extreme hyperphagia and reduced basal metabolic rate. However, POMC neurons are heterogeneous in many aspects and accumulating evidence suggests that different subpopulations of the neurons regulate separate neurological processes that together result in normal or pathological control of caloric balance. The overall goals of this project are to identify specific functions of these neuronal subpopulations and the neuroanatomic and molecular pathways that they utilize. Specific aim 1 includes a series of behavioral and neuropharmacological studies to probe the underlying component processes and neural substrates contributing to hyperphagia in POMC-deficient mouse models. These experiments utilize a newly developed method for meal pattern analysis and will test the hypothesis that melanocortin signaling coordinately modulates stereotyped motor, reward, and hedonic aspects of feeding behavior; processes which are particularly relevant to human issues surrounding food choice and meal size in the clinical pathogenesis of obesity. Specific aim 2 continues our ongoing work to define the neurochemical phenotype of POMC neurons and their collateral axonal projections to multiple brain sites. A novel retrograde tracing method involving site-specific microinjections of a canine adenoviral vector expressing Cre recombinase into mutant mice with a reversibly silenced POMC gene allele will be used to map axon collaterals to specific combinations of target sites. We will also use multilabel immunohistochemical and genetic techniques to study the unexpectedly complex dendrites of these neurons that receive synaptic inputs from distal sites. Finally, in Specific aim 3 we will use complementary genetic approaches to study the unique functional role of spatially distinct POMC neuron subpopulations or developmentally altered POMC gene expression in the prevention or mitigation of obesity. The techniques involved are aggregation chimera formation and Cre recombinase mediated reactivation of POMC expression from a neuron-specific and reversibly silenced POMC allele.